WASHINGTON (AFP) - Bradley Manning, a US soldier held for allegedly passing classified documents to WikiLeaks, is being transferred to a new prison facility after intense criticism of the conditions of his detention.

The Pentagon on Tuesday announced his imminent transfer to a Kansas military facility it said was better-suited for a long-term stay, while denying that the move was in response to criticism of his treatment in Quantico, Virginia.

"We have decided that the new joint correctional facility at Fort Leavenworth, Kansas, is the most appropriate facility for Private Manning," Jeh Johnson, the general counsel of the Department of Defense, said Tuesday.

The 23-year-old Welsh-born US army intelligence officer, who allegedly provided the whistleblowing website WikiLeaks with a trove of secret military and diplomatic documents, has been imprisoned at Quantico since July 2010.

Manning's detention conditions, which have included solitary confinement and being forced to sleep naked, have drawn the attention of Amnesty International, the American Civil Liberties Union and the British government.

In February Amnesty called Manning's detention "harsh and punitive" and urged Britain to intervene on his behalf.

And last month State Department spokesman Philip Crowley stepped down after being quoted as saying that Manning's treatment was "ridiculous and counterproductive and stupid."

Johnson initially insisted the move to Leavenworth had nothing to do with those complaints.

"We remain satisfied that Private Manning's pretrial confinement at Quantico was in compliance with legal and regulatory standards in all respects," he told reporters.

After questioning, however, Johnson said: "I won't say that his conditions at Quantico had nothing to do with this," before arguing that the Kansas facility was more appropriate for long-term stays.

"Quantico is a place where pre-trial confinees reside for one month, two months, three months. It is rare if not unprecedented that somebody is there for as long as nine or 10 months," he said.

Manning will be allowed visitors and, instead of being kept in solitary confinement for 23 hours a day, will be able to interact with other pre-trial detainees, officials said.

Lieutenant Colonel Dawn Hilton, the commander of the Kansas prison, said it was designed for long-term incarceration, unlike the facility at Quantico, and housed around 150 inmates, including eight in the pretrial phase.

"I have the capacity to hold not only the pretrial prisoners, but post-trial prisoners sentenced up to five years," Hilton said. "And with that comes all the support staff that Private First Class Manning may need."

"He'll receive open recreational time for three hours during the day, both indoors and outdoors, and he'll have the capability to interact with other pretrial inmates on a routine basis."

Johnson said the facility would be able to support Manning during the pre-trial and trial period, which could last months or even years.

He described the facility, which was opened in October, as a "state-of-the-art complex with the best and widest range of support services available to pretrial prisoners," including medical and mental health staff.

The leak of hundreds of thousands of documents, many related to the wars in Iraq and Afghanistan, infuriated top US officials, who insisted their publication was a threat to national security.